Proper centering of a semiconductor wafer is essential during wafer processing and transport in order to avoid costly errors such as nonuniform processing and/or wafer collision. During semiconductor device processing a wafer must be accurately centered on a wafer support platform in order to ensure the wafer will receive uniform processing across its entire face (e.g., uniformly deposited layers). Similarly, due to the rapid decrease in mechanical tolerencing associated with continually decreasing system footprint (i.e., the decrease of a fabrication system's lateral dimensions), a wafer must be accurately centered on the transfer mechanism in order to avoid collisions during semiconductor device transport, and in order to reduce the probability of the wafer sliding off or being dropped by the transfer mechanism. In addition some processes are sensitive to crystal orientation and require wafer alignment prior to being transferred to the processing chamber. Accordingly numerous wafer alignment and/or centering devices (i.e., positioning devices) exist.
Conventional centering devices typically comprise a rotatable platform having three or four wafer lift pins that lift a wafer off the platform and shift horizontally to center the wafer. A wafer position sensor positioned a small distance above the platform transmits information regarding the location of the wafer's center point to the wafer lift pins. In practice, because the wafer lift pins' horizontal movement is limited, the wafer lift pins may need to make multiple lift and shift operations to achieve the required movement of the wafer to achieve accurate centering. The wafer edge location is checked after each lift and shift operation to verify wafer centering was achieved, or to initiate additional wafer movement.
Like conventional centering devices, conventional wafer alignment devices typically comprise a platform having a rotatable surface and a wafer position sensor positioned a small distance above the platform. The wafer position sensor identifies when a flat/notched region of the wafer is in a predetermined position and transmits this information to the rotatable platform to cease rotation.
Because a wafer must be centered before it can be aligned, many conventional devices include both alignment and centering capabilities. However, whether for centering and/or alignment, wafer transfer to, wafer positioning at, and wafer transfer from conventional positioning devices (i.e., the wafer transfer and positioning operation) follows the same sequence: 1) a wafer handler extracts a first wafer from a multi-slot wafer carrier and transports the first wafer to the positioning device; 2) the positioning device positions the first wafer; and 3) the wafer handler returns the positioned first wafer to the multi-slot wafer carrier. Thereafter the sequence repeats and the wafer handler extracts a second wafer from the multi-slot wafer carrier and transports the second wafer to the positioning device, etc.
As indicated by the sequence described above, conventional wafer positioning devices allow only the wafer handler or the positioning device to operate at a given time (i.e., serial operation); the wafer handler remains idle while the positioning device operates, and the positioning device remains idle while the wafer handler operates. Such serial operation wastes equipment operating time, reducing throughput and increasing wafer costs. Accordingly a need exists for an improved wafer positioning device.